Cylindrical plug valves have an integral plug that rotates within a well-fitting cylindrical valve body. If a direct metal-to-metal seal is used, considerable torque is required to drive the plug valve. It can also be coated on the sealing surface of the valve body or the outer surface of the cock to reduce friction and improve sealing ability. Typical coatings include PTFE, RislanTM and ceramics. Plugs are hard-faced to reduce friction and corrosion. A PTFE sleeve can be installed between the plug and body to seal and reduce friction. Worn sleeves can be easily replaced. The use of plug valves depends on the sealing between the valve plug and the valve body to a certain extent. Cylindrical plug valves generally use four sealing methods to achieve sealing.
Oil-sealed cylindrical plug valves
The sealing of the oil-sealed cylindrical plug valve is realized by the sealing grease between the plug and the valve body (Figure 1). The sealing grease is injected into the sealing surface through the valve stem and the cock rod with a grease injection machine or a grease gun. Therefore, when the valve is in use, it can effectively make up for its lack of sealing by injecting supplementary sealing grease. Since the sealing surface is protected and doesn’t contact with the flowing medium when the valve is fully open, and the sealing surface can be sealed by the oil seal even if the sealing surface is worn; the oil-sealed plug valve is especially suitable for abrasive media. However, oil-sealed plug valves are not suitable for throttling conditions. This is because the sealing grease will be removed from the leaking sealing surface during throttling so that the sealing of the valve seat must be restored every time the valve is closed.
The disadvantage of this valve is that the addition of sealing grease requires manual operation. Although this problem can be overcome by using an automatic injection of sealing grease, it needs to increase the cost of filling equipment. Once the plug cannot rotate in the valve body due to lack of maintenance or improper selection of sealing grease, or crystallization between the seals, the valve must be cleaned and repaired.
Figure 1 Oil-sealed cylindrical plug valves
Expansion cylindrical plug valves
The sealing of the cylindrical plug valve shown in Figure 2 and Figure 3 is achieved by the expansion of the compression nut or the plug to the valve seat. The plug valve shown in Figure 2 has a packing sleeve, which is fastened to the valve body with a compression nut. Packing is usually made of compressed asbestos or Teflon. If the valve leaks, it is necessary to compress the packing again to achieve sealing, and it must be done in the closed position of the valve to prevent the packing from entering the flow channel. This valve is smaller but can be used for higher pressure and high-temperature service. The typical application of the expansion cylindrical plug valve is to isolate pressure gauges and liquid level gauges.
Figure 2 Cylindrical plug valves with compression nuts
1. Valve bodies 2. Compression nuts 3. Plugs 4. Packing
Oil-sealed cylindrical plug valves
The sealing of the oil-sealed cylindrical plug valve is realized by the sealing grease between the plug and the valve body (Figure 1). The sealing grease is injected into the sealing surface through the valve stem and the cock rod with a grease injection machine or a grease gun. Therefore, when the valve is in use, it can effectively make up for its lack of sealing by injecting supplementary sealing grease. Since the sealing surface is protected and doesn’t contact with the flowing medium when the valve is fully open, and the sealing surface can be sealed by the oil seal even if the sealing surface is worn; the oil-sealed plug valve is especially suitable for abrasive media. However, oil-sealed plug valves are not suitable for throttling conditions. This is because the sealing grease will be removed from the leaking sealing surface during throttling so that the sealing of the valve seat must be restored every time the valve is closed.
The disadvantage of this valve is that the addition of sealing grease requires manual operation. Although this problem can be overcome by using an automatic injection of sealing grease, it needs to increase the cost of filling equipment. Once the plug cannot rotate in the valve body due to lack of maintenance or improper selection of sealing grease, or crystallization between the seals, the valve must be cleaned and repaired.
Figure 1 Oil-sealed cylindrical plug valves
Expansion cylindrical plug valves
The sealing of the cylindrical plug valve shown in Figure 2 and Figure 3 is achieved by the expansion of the compression nut or the plug to the valve seat. The plug valve shown in Figure 2 has a packing sleeve, which is fastened to the valve body with a compression nut. Packing is usually made of compressed asbestos or Teflon. If the valve leaks, it is necessary to compress the packing again to achieve sealing, and it must be done in the closed position of the valve to prevent the packing from entering the flow channel. This valve is smaller but can be used for higher pressure and high-temperature service. The typical application of the expansion cylindrical plug valve is to isolate pressure gauges and liquid level gauges.
Figure 2 Cylindrical plug valves with compression nuts
1. Valve bodies 2. Compression nuts 3. Plugs 4. Packing
The cock of the valve shown in Figure 3 is divided into two parts and is stretched by an externally adjustable wedge. The sealing of the valve is achieved by a narrow PTFE ring inserted into the faces of the two plugs. This valve can also realize double cut-off and discharge due to this special structure.
Figure 3 Expansion cylindrical plug valves
This kind of valve is especially suitable for occasions that require the use of stainless steel and other precious alloys to make valves. It can be used to deal with mud, but it is not suitable for abrasive media. Medium with high viscosity tends to make the wedging mechanism difficult to operate. However, it is very reliable to use the valve on the occasion of frequent operation within the limited range of use conditions.
O-ring sealed cylindrical plug valves
The plug valve shown in Figure 4 is a cylindrical plug valve sealed with an O-ring. The O-ring is inlaid on the seam allowance of the plug to form the sealing ring of the plug. When the valve is closed, the medium flows into the chamber between the plug and the valve body through the upstream O-ring, and forces the downstream O-ring to be in close contact with the plug and the valve body sealing pair to form a seal. These valves are mainly used in high-pressure hydraulic systems. However, as long as the material of the elastic O-ring is compatible with the medium, it can also be used on other occasions.
Figure 4 O-ring sealed cylindrical plug valves
Figure 3 Expansion cylindrical plug valves
This kind of valve is especially suitable for occasions that require the use of stainless steel and other precious alloys to make valves. It can be used to deal with mud, but it is not suitable for abrasive media. Medium with high viscosity tends to make the wedging mechanism difficult to operate. However, it is very reliable to use the valve on the occasion of frequent operation within the limited range of use conditions.
O-ring sealed cylindrical plug valves
The plug valve shown in Figure 4 is a cylindrical plug valve sealed with an O-ring. The O-ring is inlaid on the seam allowance of the plug to form the sealing ring of the plug. When the valve is closed, the medium flows into the chamber between the plug and the valve body through the upstream O-ring, and forces the downstream O-ring to be in close contact with the plug and the valve body sealing pair to form a seal. These valves are mainly used in high-pressure hydraulic systems. However, as long as the material of the elastic O-ring is compatible with the medium, it can also be used on other occasions.
Figure 4 O-ring sealed cylindrical plug valves
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